A subsea blowout preventer (“BOP”) stack is used to seal a wellbore during drilling operations, both for safety and environmental reasons. As shown in FIG. 1, a lower blowout preventer stack (“lower BOP stack”) 14 may be rigidly attached to a wellhead upon the sea floor 20, while a Lower Marine Riser Package (“LMRP”) 24 is retrievably disposed upon a distal end of a marine riser 10, extending from a drill ship 12 or any other type of surface drilling platform or vessel. As such, the LMRP 24 may include a stinger 26 at its distal end configured to engage a receptacle 28 located on a proximal end of lower BOP stack 14.
In typical configurations, the lower BOP stack 14 may be rigidly affixed atop a subsea wellhead and may include (among other devices) a plurality of ram-type blowout preventers useful in controlling the well as it is drilled and completed. Similarly, the LMRP 24 may be disposed upon a distal end of a long flexible riser that provides a conduit through which drilling tools and fluids may be deployed to and retrieved from the subsea wellbore. Ordinarily, the LMRP 24 may include (among other things) one or more ram-type blowout preventers at its distal end and an annular blowout preventer at its upper end.
When desired, ram-type blowout preventers of the LMRP 24 and the lower BOP stack 14 may be closed and the LMRP 24 may be detached from the lower BOP stack 14 and retrieved to the surface, leaving the lower BOP stack 14 atop the wellhead. Thus, for example, it may be necessary to retrieve the LMRP 24 from the wellhead stack in times of inclement weather or when work on a particular wellhead is to be temporarily stopped. When work is to resume, the LMRP 24 may be guided back to and engaged with the lower BOP stack 14 so that the ram-type blowout preventers may be opened and operations continued.
The lower BOP stack 14 may include any number and variety of blowout preventers 16 to ensure pressure control of a well, as is well known in the art. In general, the lower BOP stack 14 may be configured to provide maximum pressure integrity, safety, and flexibility in the event of a well control incident. However, various electrical, mechanical, and hydraulic controls need to extend from the surface vessel 12 to the various devices of the LMRP 24 and lower BOP stack 14. In typical subsea blowout preventer installations, multiplex (“MUX”) cables (electrical) or lines (hydraulic) transport control signals down to the LMRP 24 and lower BOP stack 14 devices so the specified tasks may be controlled from the surface. Once the control signals are received, subsea control valves are actuated and (in most cases) high-pressure hydraulic lines are directed to perform the specified tasks. Thus, a multiplexed electrical or hydraulic signal may operate a plurality of “low pressure” valves to actuate larger valves to communicate the high-pressure hydraulic lines with the various operating devices of the wellhead stack.
Therefore, several and varied feed-thru components are used to carry the various mechanical, electrical, and hydraulic signals (including working fluids) from the surface vessel 12 to the working devices of the LMRP 24 and to the lower BOP stack 14. For feed-thru components that are bridged between the LMRP 24 and the lower BOP stack 14, a first mating half of the component may be located upon a distal end of the LMRP 24 and a second mating half of the component may be located upon a proximal end of the lower BOP stack 14. The first mating half and the second mating half are part of the feed-thru component. Examples of communication lines bridged between LMRPs and lower BOP stacks through such feed-thru components include, but are not limited to, hydraulic choke lines, hydraulic kill lines, hydraulic multiplex control lines, electrical multiplex control lines, electrical power lines, hydraulic power lines, mechanical power lines, mechanical control lines, electrical control lines, and sensor lines. In certain embodiments, subsea wellhead stack feed-thru components include at least one MUX “pod” connection whereby a plurality of hydraulic control signals are grouped together and transmitted between the LMRP 14 and the lower BOP stack 24 in a single mono-block feed-thru component.
Because of the many feed-thru component connections (in one application, there may be over 50 connections between the LMRP 24 and the lower BOP stack 14) that may be present between the LMRP 24 and the lower BOP stack 14, the LMRP 24 and lower BOP stack 14 have historically been constructed as unique, custom fit and/or “paired” components, wherein each LMRP 24 is manufactured to correspond to a single lower BOP stack 14 and therefore only capable of engaging with and landing to that single lower BOP stack 14. Historically, LMRPs and lower BOP stacks have been assembled on land prior to final subsea alignment and the feed-thru components have been connected to ensure that after disassembly, the mating halves of all the feed-thru components will align properly when re-assembly takes place at the job site, e.g., undersea.
However, this dry pre-assembly performed in a ground facility is time consuming and costly as the equipment necessary for lifting the LMRP 24 (which might weight more than one million pounds) is expensive, highly specialized and the workforce involved is substantial. In addition, by having to first fit the LMRP 24 to the lower BOP stack 14 on land, it will occupy a large space of the ground facility of the manufacturer, will delay the production of more LMRPs and lower BOP stacks and will also delay the delivery of the equipment to the oil extraction operator. Therefore, because of the difficulty to precisely (and repeatably) lay out and assemble feed-thru components of LMRPs and lower BOP stacks, to date, no two LMRP/lower BOP stack combinations are interchangeable, i.e., a first LMRP that mates with a first BOP stack, when disconnected from the first BOP stack, will not fit to a second BOP stack, and the other way around.
Due to the large scale of these components and the difficulty in precisely assembling undersea the LMRPs and the lower BOP stacks, even if an oil operator orders, for example, five identical LMRPs and lower BOP stacks, according to existing methods and procedures, one LMRP will correctly fit only one lower BOP stack of the five lower BOP stacks and not the remaining lower BOP stacks as one lower BOP stack is dry fit to one LMRP due to time and construction constraints, as already explained.
Disadvantageously, the custom-fitting of the LMRP 24 and lower BOP stack 14 together increases the amount of time required for the manufacturing and assembly processes. Further, in the event that an LMRP 24 or a lower BOP stack 14 requires repair or replacement, both the LMRP 24 and the lower BOP stack 14 have to be retrieved and either repaired together or replaced with a new pair of LMRP 24 and lower BOP stack 14. Formerly, if an LMRP from one distinct assembly was to be mated with a lower BOP stack from another distinct assembly (even if the distinct assemblies are of the same type and design) both “mismatched” assemblies had to be taken to a manufacturing facility to be “fitted” together.
One reason for the dry fitting of the LMRP 24 and the lower BOP stack 14 is the plural feed-thru connections that need to match each other. The feed-thru connections typically include corresponding mating halves, i.e., a first half of the feed-thru may be attached to the LMRP 24 and the second half may be attached to the lower BOP stack 14. Therefore, precision and accuracy with respect to the location of mounting holes in the frames of the LMRP 24 and the BOP stack 14 become an issue because cutting a large hole in a frame of steel that may have a thickness between 10 to 30 cm is challenging. The mounting holes on the LMRP frame and the lower BOP stack frame for a particular component may need to be positioned within a selected tolerance (hundredths to thousands of a millimeter) to allow the halves of the component to be mated to properly align and engage upon final assembly.
However, in conventional systems, due to the size of the LMRP 24 and lower BOP stack 14, fabrication limitations of the corresponding mating halves may be such that when assembled, corresponding mating halves are misaligned. Equipment that may typically be used for such precise tolerance may be unable to accommodate the large frames of the LMRP 24 and lower BOP stack 14. In this regard, it is noted that a conventional LMRP or a lower BOP stack may weight as much as one million pounds or more each and may have sizes in the order of a few yards if not tens of yards. In addition, in use, the entire process of mating is taking place undersea, where it is difficult to dispatch an operator to supervise the mating.
One approach for facilitating the connection of the LMRP and the lower BOP stack is discussed next with regard to FIGS. 2 and 3. FIGS. 2 and 3 show a hot stab line connection that is currently in use. FIG. 2 shows a hot stab feed-thru component 30 having a first half 32 and a second half 34. The two halves 32 and 34 are shown disconnected in FIG. 2. The first half 32 is fixed to a frame 36 while the second half 34 may slide a distance 44 relative to frame 38. In other words, the second half 34 may move in a plane perpendicular to a longitudinal axis 45 of the hot stab 30. However, this move is limited by a hole 46 in which the second half 34 is placed. The first half 32 includes an extension 40 which may rotate by about one degree around the longitudinal axis 45 of the hot stab 30. Prior to engaging the first and second halves 32 and 34 as shown in FIG. 3, the frame 36 and frame 38 must be in a final position so that neither frame moves relative to the other. In this regard, it is noted that both FIGS. 2 and 3 show the frames 36 and 38 being separated by a same distance, i.e., not moving relative to each other while contacting first half 32 to the second half 34. Another prior condition for engaging the first and second halves 32 and 34 shown in FIGS. 2 and 3 is that external pressure from an accumulator should be available to the first half 32 so that extension 40 can be lowered towards the second half 34 as shown in FIG. 3. The extension 40 enters the space 42 shown in FIG. 2 for engaging the second half 34 under the action of the external pressure.
Thus, the hot stab 30 shown in FIGS. 2 and 3 requires, prior to engagement of the halves 32 and 34, that (I) frames 36 and 38 are fixed in a final position, and (II) external pressure is available to contact and engage the feed-thru components to achieve the hot stab connection. One disadvantage of this type of connection is the following. Suppose that the extension 40 is extended relative to the first frame 32 such that the extension 40 extends past the first frame 36 towards the second frame 38. Given the large weight of the LMRP 24 and the lower BOP stack 14, if a misalignment occurs between the halves 32 and 34 of the hot stab shown in FIGS. 2 and 3 and the misalignment cannot be corrected by the movement of the extension 40 or the movement of the second half 34, then the extension 40 might be crashed by the weight of the first frame 32. It is noted that a typical diameter of the extension 40 is one inch (2.54 millimeters). Thus, the extension 40 is not extended unless the first and second frames are in final position, i.e., the frames do not move one relative to another.
What is needed is a simplified procedure and/or assembly for connecting an LMRP 24 to a lower BOP stack 14 without the need of a dry pre-assembly and/or pressurized extensions.